1. Field of the Invention
The present invention relates generally to ferromagnetic memory and more specifically to ferromagnetic memory utilizing giant magnetoresistance and spin polarization.
2. Description of the Background Art
For many years, random access memory for computers was constructed from magnetic elements. This memory had the advantage of very high reliability, nonvolatility in the event of power loss and infinite lifetime under use. Since this memory was hand assembled from three-dimensional ferrite elements, it was eventually supplanted by planar arrays of semiconductor elements. Planar arrays of semiconductors can be fabricated by lithography at a much lower cost than the cost of fabricating prior art magnetic ferrite memory elements. Additionally, these semiconductor arrays are more compact and faster than prior art ferrite magnetic memory elements. Future benefits of increasingly smaller scale in semiconductor memory are now jeopardized by the concern of loss of reliability, since very small scale semiconductor elements are not electrically robust.
Non-volatile magnetic memory elements that are read by measuring resistance have been previously demonstrated by Honeywell Corporation. These systems operate on the basis of the classical anisotropic magneto-resistance phenomena, which results in resistance differences when the magnetization is oriented perpendicular versus parallel to the current. Previous work by others has shown that a 2% change in resistance is sufficient to permit the fabrication of memory arrays compatible with existing CMOS computer electronics. Unfortunately, scaling of these elements down from the current 1 xcexcm size has proved challenging.
Accordingly, it is an object of this invention to produce an inexpensive non-volatile random access ferromagnetic memory.
It is another object of the present invention to produce a non-volatile ferromagnetic random access memory that is faster than the presently available semiconductor random access memory.
It is a further object of the present invention to produce a highly compact non-volatile random access ferromagnetic memory.
These and additional objects of the invention are accomplished by a non-volatile random access memory element that employs giant magnetoresistance (GMR), i.e., the spin-valve effect. The memory element has a sandwich structure in which layers of ferromagnetic material, at least one of which has its magnetic moment oriented within the plane of the layer, are spaced apart by a layer of a non-magnetic metal. Conducting leads provide current to pass through the ferromagnetic layers, perpendicular to the magnetic moment of the at least one ferromagnetic layer having its magnetic moment oriented within the plane of the ferromagnetic layer. Between and in physical contact with one of the ferromagnetic layers and the conducting leads there may be an antiferromagnetic layer. The antiferromagnetic layer fixes the direction and magnitude of the magnetic moment of the ferromagnetic layer that it contacts.
When a voltage is applied across the two ferromagnetic layers the resistance varies depending upon whether the magnetic moments of these layers are aligned in the same direction with respect to each other. Resistance between the two layers increases when the magnetic moments of these two ferromagnetic layers are not aligned in the same direction, i.e, misaligned or anti-parallel (anti-aligned). The resistance between the two layers drops when the magnetic moments of these two ferromagnetic layers are in essentially the same direction (parallel) or move from a more anti-parallel orientation to one which is more parallel. The more parallel state can be assigned a value of xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d while the more antiparallel state can be assigned, respectively, a value of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d. Thus, the alignment status of each memory element according to the present invention represents a bit of information.
The bit can be altered in a memory element according to the present invention by applying an sufficiently high current in the conducting leads in order to generate a magnetic field sufficient to align, in one direction, the magnetic moment of any unpinned ferromagnetic layer along one of the easy directions of orientation. The direction of orientation favored by the orienting current is of course determined by the polarity of that orienting current. Once set, the bit may be read by applying a smaller current through the appropriate conducting leads and determining whether the resistance is more or less than that of a reference resistance.